"There are certain crimes that are so beyond the pale that I understand society's need to express its outrage," Obama said. "So I have not traditionally been opposed to the death penalty in theory. But in practice it's deeply troubling."

He added, "All of this, I think, has led me to express some very significant reservations."

Obama told Keller that he is withholding action for now, and will wait until the results of a Department of Justice review of the death penalty is released before deciding if the administration will act to curb the use of capital punishment.

The president's comments come as the Obama administration continues to push for broader criminal-justice reform.

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Barack Obama portraits through the years

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Obama: The death penalty is 'deeply troubling'

2009

In this photo provided by the Obama Transition Office, U.S. President-elect Barack Obama poses for an official portrait on January 13, 2009 in Washington, DC.

(Photo by Pete Souza/Obama Transition Office via Getty Images)

2010

US President Barack Obama speaks in the Grand Foyer following a meeting with senior intelligence officials and cabinet members January 5, 2010 at the White House in Washington, DC.

(Photo by Mandel Ngan/AFP/Getty Images)

2011

US President Barack Obama delivers remarks at the Rose Garden of the White House in Washington, DC on November 7, 2011 on tax credits included in the American Jobs Act and new executive actions that will help get veterans back to work.

(Photo by Jewel Samad/AFP/Getty Images)

2012

In this handout from the White House, official portrait of U.S. President Barack Obama in the Oval Office on December 6, 2012 in Washington, D.C.

(Photo by Pete Souza/The White House via Getty Images)

2013

US President Barack Obama announces his nominee for the Federal Housing Finance Authority, North Carolina Democrat Representative Mel Watt, and his nominee for the Federal Communications Commission (FCC), venture capitalist Tom Wheeler, in the East Room of the White House in Washington, DC, on May 1, 2013.

(Photo by Saul Loeb/AFP/Getty Images)

2014

U.S. President Barack Obama hosts a St. Patrick's Day reception for Prime Minister Enda Kenny of Ireland and his wife Fionnuala O'Kelly in the East Room of the White House March 14, 2014 in Washington, DC.

(Photo by Ron Sachs-Pool/Getty Images)

2015

US President Barack Obama speaks at an event marking the 5th anniversary of the Affordable Care Act in Washington, DC, March 25, 2015.

(Photo by Jim Watson/AFP/Getty Images)

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The White House, along with members of Congress from both parties, views criminal-justice reform as a rare space where compromises over issues such as prison overcrowding and mass incarceration can be reached.

Earlier this week, a bipartisan criminal-justice reform bill passed the Senate Judiciary Committee, clearing a key hurdle that could help bring a final bill closer to passage. The legislation addresses mandatory minimum sentences, allowing some prisoners convicted of nonviolent crimes to have their cases reexamined in certain circumstances.